Patent Document 1 relates to a control method for suppressing periodic disturbances. Patent Document 1 discloses a method for suppressing torque ripples, which are periodic disturbances, in an electric motor, by means of a “periodic disturbance observer method”.
Patent Documents 2 and 3 achieve stable and high-speed control by suppressing resonance and disturbances by means of robust control using μ-synthesis. In particular, the structure of a dynamo motor (engine bench system) drive power measurement device is described.
The present invention simultaneously achieves resonance suppression and periodic disturbance suppression in a system. Herebelow, the invention will be explained by describing the case of a multi-inertia motor drive system such as a dynamometer system, but the possible applications are not limited thereto. For example, in the case of an electric power grid system, the invention can simultaneously achieve grid resonance suppression and harmonics suppression.
The resonance and disturbances that occur in a control system not only reduce the control performance, but also cause the system to become unstable. When system resonance and disturbances cannot be adequately reduced by means of structural measures due to restrictions on structure, specs, cost or the like, it has been common, for a long time, to implement resonance suppression control and disturbance suppression control.
Disturbances include “non-periodic disturbances” that occur in all frequency bands, such as white noise, and “periodic disturbances” that occur periodically in specific frequency components. For example, in the case of a motor, torque ripples correspond to periodic disturbances, and in the case of an electric power grid, harmonics and the like correspond to periodic disturbances. In particular, since periodic disturbances occur repeatedly in specific frequency components, if they match the resonance frequency of the system, they can be amplified to become extremely large disturbances, in which case they will have a serious impact on failure and instability in the system.
As a general method for suppressing disturbances, disturbance observers are widely used. Disturbance observers estimate disturbances by using command values and inverse properties of a control-implemented model, and remove the disturbances by subtracting the estimated disturbances from the command values.
However, the inverse properties of the control-implemented model generally have derivative elements, so a low-pass filter is used for preventing gain amplification in high frequency bands and forming pseudo-derivatives. There is a problem in that disturbances in some frequency bands cannot be estimated and removed due to the presence of this low-pass filter.
For example, the torque ripples in a three-phase motor are periodic disturbances that occur in synchronization with the rotation speed, and are known to occur mainly at multiples of 6×n of the rotation speed. As the rotation speed becomes higher, the frequency components of the torque ripples also become higher and enter into the control band, in which the periodic disturbances cannot be adequately suppressed by disturbance observers. Furthermore, when there is a system resonance frequency in a high frequency band, the periodic disturbances are amplified, generating extremely large vibrations and noise.
Patent Document 1 focuses on the periodicity of periodic disturbances, and discloses a “periodic disturbance observer method” in which the disturbance observer is generalized in a rotating coordinate system that is synchronized with the torque ripple frequency.
In this method, the control system only affects periodic disturbances in specific frequency components, and does not include derivative properties in the inverse properties of the control-implemented model, so the method can be used to suppress periodic disturbances even in high frequency bands. However, it cannot be used to suppress non-periodic disturbances outside the specific frequency components. Therefore, for example, if there is a non-periodic disturbance at the same frequency as the system resonance frequency, the effects thereof cannot be removed, so it is necessary to combine said method with other measures for suppressing non-periodic disturbances.
Patent Documents 2 and 3 use robust control by means of μ-synthesis as an example of a resonance/disturbance suppression method. With such robust control, the resonance and disturbances in systems are suppressed comprehensively, and the controller is designed to achieve a command value response and disturbance response that are desired.
However, when perturbations in various parameters over a wide range of frequency bands are taken into consideration, the design tends to become conservative. Additionally, in accordance with Bode's theorem, there is a tradeoff between the resonance suppression performance and the quickness of the command value response and the disturbance response. For this reason, there are cases in which the disturbance response properties near the resonance frequency exceed 0 dB, and if there is a large periodic disturbance at that frequency, the resonance may not be able to be adequately suppressed.
As indicated above, a problem in control systems is the matter of how resonance, non-periodic disturbances and periodic disturbances can all be suppressed in order to raise the system control performance.